Bill,
With pre-defined store.groupers this works well.
However, I would like to be able to allow the user at run-time to choose what fields to group by. If they choose "Group by this column" then I can only get one field grouping. Is there any way to allow an ADD rather than a REPLACE?

It worked for me now... i m not getting summation correct. I need to set following features:
features: [{groupHeaderTpl:'{name}', enableGroupingMenu : false, enableNoGroups: false, ftype:'multigrouping'}] instead of ftype:'multigroupingsummary'
Investigating more for the summation. thanks for nice api